Fix missing DECLARE_GLOBAL_DATA_PTR on CONFIG_LPC2292 in serial

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/arm720t/serial.c b/cpu/arm720t/serial.c
index 27eb73a..1b0e147 100644
--- a/cpu/arm720t/serial.c
+++ b/cpu/arm720t/serial.c
@@ -125,6 +125,8 @@
 
 #elif defined(CONFIG_LPC2292)
 
+DECLARE_GLOBAL_DATA_PTR;
+
 #include <asm/arch/hardware.h>
 
 void serial_setbrg (void)